On the video, I didn't see him make troops "sling" weapons. I saw him getting people to point weapons down.
There is a reason for this. The reason is so that if a gun goes off unintentionally, the bullet goes into the ground instead of into the guy next to you. If the gun is pointed up, the bullet may not hit the guy next to you, but it might hit somebody's kid on the second floor of the house you're standing next to (they were in an urban area).
This is the reason why troops are supposed to point their guns DOWN. On Iraq footage, you see troops on patrol, they are pointing their guns down except when actually firing at an enemy
In the clip I saw, he was clearly yelling "sling 'em."